The importance of power quality has been marginalised in
recent years, according to Steve Hughes of REO UK. He
believes that few significant energy users understand that a
welcome side effect of power quality improvement is a
reduction in the cost of electricity

Apower quality issue is generally
defined as a voltage, current or
frequency deviation in the applied
mains that has a detrimental effect on the
performance or longevity of connected
equipment. Usually, if the voltage or current is
not a pure sine wave then there is a problem.

In the UK the useful power supplied to a
system is at a frequency of 50Hz and at unity
power factor – anything that deviates from
this will result in higher costs. So if
additional harmonic currents are being
drawn, users will pay for them in cash. If a
site has a low power factor, it is likely they
will be paying for that too.

It is easy to blame the power utility
companies for quality issues on their
network, but more often than not, it is the
energy user’s own equipment and that of
their neighbours that is the root cause of the
problem. Almost anything that is connected
to the mains network can cause difficulty,
but components and systems that switch
high power at low frequencies such as power
factor correction (PFC) capacitor banks,
thyristor-controlled equipment like heater
banks and the DC input sections of variable
speed drives (VSDs) create the most issues.

Such devices can cause sags, swells,
transients and harmonic issues which affect
equipment performance and longevity, but it
is questionable whether users are bothered
by this. The likelihood is that they are not;
provided their equipment and site is running
satisfactorily, despite the known benefit of
increased power quality.

Harmonic mitigation
For example, most VSD manufacturers will
recommend that users deploy some element
of harmonic mitigation, and that will
typically involve an iron core mains choke.

An input choke will effectively smooth out
these harmonics to make the current
waveform more sinusoidal and less
harmonic-laden. This is demonstrated in a
Fourier analysis of harmonic content with
and without mains chokes.

REO makes single-phase and three-phase
line chokes with one, two and four per cent
inductance, allowing OEMs to make
engineering decisions to mitigate harmonics
based on the application and environment.

In an ideal world they would actually
measure the harmonics and select a choke
accordingly, but more often than not, they
would select a 2% choke with the
understanding that this offers the best
performance/price trade-off.

It is worth mentioning that input chokes
and mains RFI filters are typically discrete
components. However, as a manufacturer,
REO can easily integrate them as a single
choke-filter combination, which can be
water-cooled and even added to an output
choke or brake resistor, so the customer does
not need three separate parts. In addition, its
products have ingress ratings up to IP64.

Increased ingress protection is often needed
where the equipment is to be used in a harsh
environment or where there are space
constraints.

Site surveys
Part of REO’s day-to-day activities involves
carrying out site surveys and making
recommendations on power quality based on
the findings. We were recently asked to look
at a site that was experiencing some
reliability issues. The site was using a large
number of high current DC power supplies,
comprising thyristor bridges in a typical B6
configuration.

The phase current was approx 1500A, but
this comprised a number of higher order
harmonics. REO determined that this was
the first issue that should be addressed and
subsequently designed, specified,
manufactured and fitted appropriate
harmonic filtering. The result was two-fold:
an increase in power quality with an
improvement in process reliability, but also a
500A reduction in metered current. For a site
that operates continuously the payback on
the filtering equipment is many times over.

This example is perhaps one of the clearest
that could be produced to illustrate the cost
savings that go hand in hand with power
quality engineering. A 500A reduction in
metered current on a 1500A application
represents a cost reduction of one third.

Imagine combining this kind of saving with
more conventional energy reduction
methods? So, instead of thinking of the cost
of power quality, think instead of saving
money and improving power quality into the
bargain.